The present invention relates in general to cardiac stimulation devices, such as pacemakers, defibrillators, cardioverters, implantable cardioverter-defibrillators (xe2x80x9cICDsxe2x80x9d), and similar cardiac stimulation devices that are capable of monitoring and detecting electrical activities and events within the heart. In particular, this invention pertains to a system and method for optimizing the post-ventricular atrial blanking period of an atrial channel of an implantable dual chamber stimulation device, by implementing a segmented post-ventricular atrial blanking period and properly identifying far-field signals sensed on an atrial channel.
Implantable medical devices, such as pacemakers, defibrillators, cardioverters, and implantable cardioverter-defibrillators (xe2x80x9cICDsxe2x80x9d), collectively referred to herein as implantable cardiac stimulating devices, are designed to monitor and stimulate the heart of a patient who suffers from a cardiac arrhythmia. Using leads connected to a patient""s heart, these devices typically stimulate the cardiac muscles by delivering electrical pulses in response to measured cardiac events that are indicative of a cardiac arrhythmia. Properly administered therapeutic electrical pulses often successfully reestablish or maintain the heart""s regular rhythm.
Implantable cardiac stimulating devices can treat a wide range of cardiac arrhythmias by using a series of adjustable parameters to alter the energy, shape, location, and frequency of the therapeutic pulses. The adjustable parameters are usually defined in a computer program stored in a memory of the implantable device. The program, which is responsible for the operation of the implantable device, can be defined or altered telemetrically by a medical practitioner using an external implantable device programmer.
Programmable pacemakers are generally of two types: (1) single-chamber pacemakers, and (2) dual-chamber pacemakers. In a single-chamber pacemaker, the pacemaker provides stimulation pulses to, and senses cardiac activity within, a single-chamber of the heart, either the right ventricle or the right atrium. In a dual-chamber pacemaker, the pacemaker provides stimulation pulses to, and senses cardiac activity within, two chambers of the heart, namely both the right atrium and the right ventricle. The left atrium and left ventricle can also be sensed and paced, provided that suitable electrical contacts are effected therewith.
One problem faced with the advent of dual-chamber pacemakers is that when a pacemaker delivers a stimulation pulse to the ventricle during an appropriate portion of a cardiac cycle, this pulse would be sensed by the atrial channel. Therefore, it is a common practice in the art to apply a post-ventricular atrial blanking (PVAB) period upon delivery of a ventricular stimulation pulse, in order to prevent the saturation of the sense amplifiers of the atrial channel. Because ventricular and atrial responses are sensed through the same lead electrodes through which the stimulation pulses are delivered, the resulting polarization signal, also referred to as an xe2x80x9cafterpotentialxe2x80x9d, formed at the electrodes, can corrupt the evoked response which is sensed by the sensing circuits. This undesirable situation occurs often because the polarization signal can be three or more orders of magnitude greater than the evoked response.
Furthermore, the lead polarization signal is not easily characterized; it is a complex function of the lead materials, lead geometry, tissue impedance, stimulation energy and other variables, many of which are continually changing over time. By disabling the atrial sense amplifier, that is applying a refractory or xe2x80x9cblankingxe2x80x9d period, upon the delivery of a ventricular stimulating pulse, the atrial sense amplifier is not affected by the ventricular stimulation pulse. At a specified time interval after the delivery of a ventricular stimulating pulse, the atrial sense amplifiers are enabled again to sense intrinsic or evoked atrial events.
However, the PVAB period poses a new problem in that it may occur mid-way or even late in the atrial cycle and may therefore result in an atrial channel inability to sense the next intrinsic atrial event. Essentially, the atrial channel is xe2x80x9cblindedxe2x80x9d to rapid atrial rates precluding proper diagnostic and therapeutic measures by the implanted cardiac device. Instead, a missed atrial event would trigger an atrial stimulation pulse to be inappropriately delivered by the pacemaker. Such inappropriate pacing could endanger the patient by inducing a sequence of events that might induce cardiac arrhythmias.
Another problem faced with the development of dual-chamber pacemakers is that the evoked R-wave (the electrical signal associated with ventricular contraction) subsequent to a ventricular stimulation pulse will typically propagate to the atrium in patients with intact atrioventricular (xe2x80x9cAVxe2x80x9d) conduction. This propagated signal of a ventricular R-wave in the atria is commonly referred to as a xe2x80x9cfar-field R-wavexe2x80x9d (FFR). Even a premature ventricular contraction (PVC), an arrhythmic event common in many patients requiring implantable cardiac devices, can propagate and produce a far-field signal on the atrial channel. Such far-field signals sensed by the atrial channel could be interpreted as atrial events. This erroneous sensing could easily be misinterpreted by the pacemaker""s controlling operations as a change in atrial rate or even an atrial arrhythmia and consequently invoke improper therapeutic measures, potentially harming the patient.
In order to overcome this risk, the post-ventricular atrial blanking period employed upon the delivery of a ventricular pulse is commonly programmed long enough to encompass the far field signal associated with the propagation of a ventricular R-wave subsequent to a ventricular stimulation pulse. This post-ventricular atrial blanking period is commonly programmed to be a fixed time interval, typically 150 msec.
However, this relatively long, fixed post-ventricular atrial blanking period can exacerbate the limitations of a dual-chamber device in that the ability of the pacemaker to detect high atrial rates may be further impaired.
The window of time that the atrial channel is enabled for sensing atrial events is directly reduced as the post-ventricular atrial blanking period is lengthened to eliminate far-field signals from being sensed. Furthermore, conduction time between the ventricle and atrium will vary from patient to patient. In some patients, far field signals associated with ventricular events may occur even later than the typically programmed 150 msec blanking period. Using still longer blanking periods could more severely impair the pacemaker""s ability to detect even normal atrial rates.
It would thus be desirable to provide a system and method for automatically adjusting the post ventricular atrial blanking period such that the blanking period following a ventricular stimulation pulse is minimized, thereby allowing the longest atrial sensing window possible in an implantable dual chamber stimulation device. Furthermore, it would be desirable to implement the system and method in a way that allows far-field signals sensed by the atrial channel to be properly interpreted as the ventricular events that they are associated with, thereby excluding them from atrial rate determinations. It would further be desirable to enable the pacemaker to perform this automatic post-ventricular atrial blanking period adjustment without requiring dedicated circuitry and/or special sensors.
The features of the present invention address the limitations and disadvantages discussed above. In accordance with the present invention, a system and method are provided for automatically adjusting the post-ventricular atrial blanking (PVAB) period of the atrial sensing channel of an implantable dual chamber stimulation device. The PVAB period will be segmented so as to maximize the atrial sensing window while eliminating the possibility of a far-field ventricular R-wave (FFR) from being detected on the atrial sense channel.
The system and method of the present invention overcome the limitations of atrial sensing associated with long refractory periods normally practiced in the art by providing for a considerably shortened blanking period upon delivery of a ventricular stimulation pulse followed by a far-field interval (FFI) window centered around a FFR signal. This method does not require special dedicated circuitry or special sensors to implement the automated procedure. All of the aforesaid advantages and features are achieved without incurring any significant disadvantage.
The present invention provides an implantable medical device, hereinafter referred to as pacemaker for simplicity purposes, which is equipped with cardiac data acquisition capabilities. A preferred embodiment of the pacemaker includes a control system for controlling the operation of the pacemaker, a set of leads for receiving atrial and ventricular signals and for delivering atrial and ventricular stimulation pulses, a set of sensing circuits comprised of sense amplifiers for sensing and amplifying the atrial and ventricular signals, a sampler, such as an A/D converter, for sampling atrial and/or ventricular signals, and a pulse generator for generating the atrial and ventricular stimulation pulses. In addition, the pacemaker includes memory for storing operational parameters for the control system, such as atrial or ventricular signal sampling parameters, and atrial or ventricular signal samples, and also includes a telemetry circuit for communicating with an external programmer.
In a preferred embodiment, the pacemaker control system sets a segmented post-ventricular atrial blanking (PVAB) period and automatically adjusts this segmented PVAB period as necessary. The segmented PVAB period includes two segments. The first segment is an absolute blanking period applied upon the delivery of a ventricular stimulation pulse, and the second segment is a far-field interval (FFI) window centered on a far field signal such as a FFR signal.
The first segment disables the atrial sense amplifier in the event of a ventricular pulse to avoid corruption of the atrial sense amplifier after ventricular pacing pulses. This period of xe2x80x9cabsolute blankingxe2x80x9d in the present invention is considerably shorter than that practiced in the prior art. The possibility to shorten this absolute blanking period without posing undue risk to the patient is achieved by the implementation of the second segment of the PVAB period, i.e., the FFI window. If an event is detected outside the FFI window (i.e., an alert period), it is automatically presumed to be an atrial event (i.e., a P-wave).
During the FFI window, the atrial sense amplifier is enabled to sense, but any signal occurring during this FFI window is classified as a non-atrial event, i.e., a far-field wave (either a far-field R-wave or a far-field T-wave). Alternatively, any such event can be evaluated further by the pacemaker control system, in order to verify whether this signal is an actual atrial event or indeed a far-field wave. The temporal location and duration of the FFI window may be set manually by a medical practitioner, or automatically through a program stored in the pacemaker""s memory.
In one embodiment of the invention, a signal occurring within the FFI window is verified as a far-field R-wave by comparing it to a signal recognition template automatically produced by the pacemaker. The pacemaker produces this template by delivering a ventricular stimulation pulse and then sampling a resulting far-field R-wave that is expected to follow a successfully captured ventricular stimulation pulse. By sampling and averaging several such FFR waves, a signal recognition template of a FFR signal is produced. A signal received by the atrial sensing channel during the FFI window can therefore be compared to the far-field signal recognition template. If the FFI signal approximately equals the template, the pacemaker operation recognizes the signal as a non-atrial event. If an intrinsic ventricular event is sensed, the stimulation device does not initiate the PVAB period but rather sets the FFI window.
The system and method of the present invention thus automatically set a segmented post-ventricular atrial blanking period, and when necessary, verifies if a sensed atrial signal is a far-field signal associated with a ventricular event. This is accomplished without requiring dedicated or special circuitry and/or sensors. The system and method allow reliable sensing of atrial events in dual-chamber pacemakers where after-potentials and far-field signals limit reliable detection of atrial rhythm. In addition, the system and method enable dual-chamber pacemakers capable of automatic mode-switching to automatically adapt the PVAB period to allow sensing and appropriate classification of far-field signals during automatic mode-switching.